Documents such as cheques, vouchers, passports, certain gift vouchers, identity documents and various papers such as medical prescriptions, title deeds and various civil status documents contain personal information susceptible to alteration by forgers.
The alterations consist of using erasing agents to remove the variable data on the documents and affixing new information, for example a new recipient or a different amount on a cheque.
The erasing agents used by forgers are all those likely to be found commercially; in particular bleach, acids, bases, oxidation-reduction combinations, solvents are used and, very generally, all products capable of erasing inks such as for example hair sprays, white spirit, brake fluid, liquid stain removers, etc.
Among the known products that cannot be forged, there may be mentioned in particular those described in patent FR 2 650 606 which make it possible to treat papers with respect to bases, eraser pencils and alkaline reducing agents or patent EP 190 087 which more specifically indicates reagents for eraser pencils. Document EP 494 828 indicates a reaction for unforgeability with respect to sodium sulphite. Documents U.S. Pat. No. 4,478,681, EP 174 885, EP 378 029 or EP 632 162 indicate reagents with respect to oxidation-reduction combinations and papers that cannot be forged treated in this way. Reagents with respect to acids have been known for a very long time, for example from U.S. Pat. No. 322,130 dated 14 Jul. 1885 or U.S. Pat. No. 643,084 from 1900.
Reactions for unforgeability with respect to solvents have been known in security papers for several years according to different operating methods:                Operation by printing coloured inks containing dyestuffs that are soluble in solvents which stain the paper during an attempt to erase the inks, these dyestuffs are generally introduced into a security background printed in microtext or into a background image on the parts intended to be written on by hand. The process of treating the papers according to this operating method is a printing process.        Operation by solubilization of dyestuffs that are not soluble in water but are soluble in solvents, these so-called “organo-soluble” dyestuffs being finely dispersed in the bulk of the paper, the particles being typically less than 20 μm. The process of treating papers according to this operating method is a papermaking process, the particles being introduced into the paper pulp before the production of the sheet and therefore distributed over its volume.        